A massive memorial celebrating the lives and legacies of Carrie Fisher and her mother, Debbie Reynolds, is being held this weekend, and one of Fisher's close friends, musician James Blunt, composed a special song dedicated to the late Star Wars icon that will play alongside a video tribute.

On Wednesday, Debbie's son, Todd Fisher, sat down with ET's Nancy O'Dell, where he opened up about the special song and his sister's friendship with the "You're Beautiful" singer.

WATCH: Carrie Fisher Once Said She Wanted Harrison Ford to Sing at Her Oscars In Memoriam Tribute

"[Years ago], James Blunt came to Los Angeles, did not have anywhere to stay and was going to come and record a [song] he was still writing," Todd said, recalling how his sister invited him to crash at her house while he wrote.

"She said, 'Stay at my place,' [so] he is literally staying at her place [when] he writes, 'You're Beautiful' in her bathroom. Because there is a piano in [her] bathroom."

"They inspired each other," Todd explained. "There was this mutual inspiration going on. She had a way of rubbing off on people and inspiring people and she inspired him."

WATCH: Todd Fisher Says Mom Debbie Reynolds 'Asked for Permission' to Go After Sister Carrie's Death

According to Todd, Blunt's new song, dedicated to Carrie, is about "the love that they had."

"It is quite special," he shared. "I actually think it is one of his best songs."

"The last lyric in the song, it's a beautiful song, is, 'I'm here to let you know. I'm here to let you go.' And it's powerful," Todd revealed. "The simplicity of his songs is pretty amazing. It's not like it's [got] complicated lyrics but something about how he strings all this together is beautiful."

Todd explained that Blunt's song doesn't yet have a name, "which shows you how fresh this is."

WATCH: Billie Lourd Pays Tribute to Mother Carrie Fisher With Heart-Wrenching Flashback Photo

"I believe this song is going to be a really big song, not because of what it is but because it is a beautiful standalone song and I think it's going to mean a lot to a lot of people who have lost love ones," Todd said. "It is truly a gift to all of us for all of us that have lost love ones… Everybody has lost somebody and this song is a true help in letting go."

Fisher also revealed a few details about the memorial service itself, including some of the night's special guests, and opened up about how Carrie's daughter, Billie Lourd, has been coping with her devastating loss. Watch the video below to hear more.

The memorial will be held on Saturday and live-streamed on DebbieReynolds.com.